The concept of handedness refers to the arm with which the player throws (right-handed or left-handed). It helps us to refine our positioning and allows us to “play” with the thrower. If the thrower is left-handed, we can position ourselves in front of the thrower’s arm to force him to throw a fall-away throw.
At a high level it is very common to see goalkeepers in 7-metre throws positioned clearly in front of the thrower’s arm, rather than in the centre of the goal.
It also serves as a bridging concept to the work of observing the arm position. If the thrower has his arm position wide, the throw will go to the side of the handedness (right-handed to his right). If he has his dominant arm too high, he may want to fall-away (right-handed to his left). It is not a definitive concept, but it serves as an approximation.